From my experience, new != better, necessary.
Example being that I replaced my hop-up as well. Went from the standard SiG552 hop-up to a G3 full-metal hop up (Yes it fits before I get a bunch of myth-sayers). Every time I've tried out the G3 hop-up unit against the stock hop-up, the stock hop-up always is superior. The G3 always seems to double feed or jam up at times.
Like I said, this is from my experience. Just keep in mind that new != better. Sometimes sticking with what works is best.
Now if you're talking about the hop-up rubber, well that's a different story. The stock one you have may be a bit stiff and you may need something a bit softer. I hear the Guarder clear hop-up sleeves are miracle workers. But judging by the fact that you said "spring", I'm going to say you're talking about the hop-up. I don't know of any sleeves that have a spring in them.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1